Process of and apparatus for producing pulp



July 16, 1935.

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PROCESS 0F AND APPARATUS FOR PRODUCING PULP STEAM Own. ET

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PROCESS OF AND APPARATUS FOR PRODUCING PULP Filed May 5, 1932 2Sheets-Sheet 2 Patented July 16, i935 UNITEDr STATES PROCESS F ANDAPPARATUS Fon' PnoDUclNG PULP William Brubacher, Kapuskasing, Ontario, l

Canada Application May 5, 1932, Serial No. 609,519

18 Claims.

This invention relates to the production of pulp from brous material,such as wood chips, saw dust, rags, old paper, straw hemp and the like,and more particularly to an improved process of and apparatus for thecontinuous cooking of such fibrous material to provide for thecontinuous production of pulp therefrom.

The principal object of this invention is to provide an eicient,economical, and commercially practical ,process of producing pulpcontinuously wherein the cooking of the fibrous material may be carriedout in a single cooking tower or digester.

An important object of the invention is to provide a process ofproducing pulp wherein the cooking of fibrous material is moreeffectively and rapidlyaccomplished than in similar processes heretoforeemployed.

Another object of the invention is to provide a process of producingpulp wherein the cooking of the fibrous material is effected with lessWaste of steam, chemicals, and raw materials than in processesheretofore employed.

A further object of the invention is to provide an efficient andcommercially practical apparatus for carrying out the process abovereferred to.

Other objects and advantages of the invention will become apparentduring the course of the following description.

While there have heretofore been proposed processes for the continuousdigesting of fibrous material, such as by the use of a. plurality ofinterconnected digesters, the customary methods of digesting brousmaterial all involve vbatch treatment. Batch digesting is a relativelyslow procedure, involves an unnecessary amount of handling .ofmaterials, and requires a battery of expensive digesters for a practicalcommercial plant. Moreover, in general practice, the materials to beemployed are brought in contact with each other in a digester attemperatures much below the boiling point of the cooking liquor, thusrequiring a considerable additional amount of i heat in order to bringthe mass in the digester to l the elevated temperature necessary foreffective i cooking of the brous material. In most instances this heatis supplied by injecting steam directly into the cooking vessel, withthe result l "o that considerable dilution of the cooking liquor takesplace, thereby reducing its eiciency. This Y l l direct heating must beclone slowly in order to prevent boring of the chips or other fibrousmaterial, but even under these conditions, a. consid erable escaping ofsulfur dioxide and precipitation of monosulite on account of localoverheating cannot be avoided. In other instances, the cooking liquor ispreheated before entering the cooking vessel, and steam is thenintroduced directly into the vessel resulting in the above men- 6 tioneddisadvantage. In some processes steam coils inside the vessel areemployed for heating indirectly the mass to be cooked, and in otherprocesses heat exchangers located outside the cooking vessel areemployed whereby the cooking 10 liquor is circulated through the heatexchangers and is heated indirectly, while at the same time steam isinjected directly into the vessel in order to bring the mass up toboiling.

As will be apparent from the foregoing, in all of the processesheretofore generally employed the fibrous material and cooking liquorare brought in contact with each other at the beginning of the processbefore either has reached the boiling temperature of the cooking liquor.Hence, it is readily apparent that in these prior processes considerabletime and a substantial amount of steam areemployed before the actualcooking stage of the process begins. In addition to the amount of timewhich is thus practically wasted, the introduction of the large amountof steam employed to bring the mass to a boiling temperature results inthe reduction of Strength of the cooking liquor and attendantdisadvantages, viz. reduced yield and lower strength of product.

In addition to the foregoing disadvantages, the majority of priorprocesses have not provided best commercial practice with respect to therecovery of waste heat and chemicals. For example, the cold cooking acidemployed in these processes is ordinarily preheated in the acid supplytank with a resulting separation of gas and a consequent weakening ofcooking liquor which is very undesirable. In other words, the fullinitial concentration of the cooking liquor is not preserved for theactual cooking operation. Moreover, in 'prior processes the raw materialto be treated is brought to the cooking temperature by added heat whilein contact with the cooking liquor, thereby necessitating theintroduction of sufllcient steam not only to bring the cooking liquor toa boiling temperature but also to heat a large mass of fibrous material.This involves an input of steam which is in excess of the amount which Ihave found is necessary if the mass of brous material is graduallybrought to the necessary temperature by the hot gases discharged fromthe zone of actual cooking of the material.

In an effort to overcome the above mentioned disadvantages, I havedevised a process in which the undesirable features of priorprocessesare not present. This process is particularly adapted for usein the production of suliite pulp and the process will be described inconnection with the production of this type of pulp. However, it will beapparent to those skilled in the art that the process is not limited tothe sulfite method.

In the accompanying drawings I have shown a form of apparatusparticularly suitable for use in the practice of my process. In thisshowing,

Figure 1 is a diagrammatic view of the assembly of apparatus preferablyemployed,

Figure 2 is a central vertical section of the assembly of the lowerportion of the digester employed, parts being shown in elevation, and

Figure 3 is a horizontal section taken substantially on line 3-3 ofFigure 2.

Referring to the accompanying drawings, the reference numeral I0designates the digester or cooking vessel which is relatively elongated.In commercial practice, this digester may conveniently be approximatelyone hundred feet in length. As shown in the drawings, the digester I0gradually tapers outwardly from the top to a point below the centerthereof and, from such point tapers more sharply inwardly toward thebottom, the extreme lower portion of the digester thus being offrustro-conical shape. As will be apparent, the walls of the digester I0should be of suitable conventional construction to withstand thereactions resulting from the cooking of fibrous material therein.

The upper end of the digester I 0 is provided with a hopper I I ofconventional design by which wood chips or other fibrous material arefed to the digester. The supply of brous material introduced into thedigester is controlled by two worms I2 and I3 which are independentlydriven by telescoping shafts I4 and I5 from suitable sources of power(not shown). The two worms are spaced a substantial distance from eachother providing a chamber or space I6 therebetween. As shown in Figurel, the upper worm I2 has a sharper pitch than the lower worm I3.

The extreme lower portion of the digester I0 is provided with aremovable bottom I 1 which has a central opening through which extends ahollow shaft I8 supported in suitable bearings and adapted to be rotatedby means of the gear I9 meshing with the gear 20 driven by the shaft 2Iconnected with a suitable source of power (not shown). The upper end ofthe hollow shaftr I8 carries a. hollow scraping member 22 which isprovided on the upper face thereof with steam discharge openings 23. Theinterior of the scraper arm 22 communicates with the interior of thehollow shaft I8 through openings 24. The upper end of the shaft I8 isclosed by means of a suitable cap 25. Packing members 26 are arrangedaround the hollow shaft I8 adjacent the bottom I'I. The interior of theshaft I8 is adapted to receive steam from a steam inlet pipe 2'I throughopenings 28, the steam inlet pipe 21 and the shaft I8 being providedwith suitable connections and packing members 29 at their junctionpoint.

The removable bottom plate I'I of the digester I0 is provided at oneside thereof with a discharge opening 30'which communicates with a pulpdischarge pipe 3| provided with a control valve 32 and connectingthrough the T 33 with the pipe 34. The T 33 is provided with a cleanoutplug 35. As shown in Figure 1, the pipe 34 is provided with a controlvalve 36 adjacent the T 33.

The lower frustro-conical portion of the digester I0 is provided on theinterior thereof with a perforated liquid distributor ring 31 which isconnected to the pipe 38. Below the ring 31 are arranged steam coils 33connected to a steam inlet pipe 40 and a steam outlet pipe 4I. The coils39 are covered by a shield 42 to prevent the lodging of pulp thereon.The pipe 40 is provided with an automatic valve 43 which is operated bya conventional thermostatic control device 44 connected to a heatresponsive member 45 arranged within the interior of the digester IIInear the bottom thereof.

The extreme upper portion of the digester I0 is provided with aconventional gas collector 46 which is connected by the pipe 41 withcooling coils 48. From the coils 48, liquid passing therethrough isadapted to be delivered by the pipe 49 to an acid reclaiming tank 50.The upper portion of the tank 50 is connected by the pipe 5I with aliquid pump 52 which is connected with the acid storage tank 53. Thelower portion of the tank 50 is connected by a pipe 54 with a liquidpump which is adapted to force liquid through the pipe 56 into theinterior of a heat interchanger 51 surrounding the extreme upper portionof the digester I 0. The heat interchanger 5'I is connected by a pipe 58with condensing coils 59 arranged in the extreme upper portion of anelongated, preferably round, pulp receiver designated generally by thenumeral 60.

The mass of material treated in the digester I0 is dischargedcontinuously through the opening 30 and conveyed to the receiver 60 bythe pipe 34 which extends into the interior of the receiver andterminates at a substantial distance from the bottom thereof. As shownin Figure l, within the lower portion of the receiver 60 is arranged aheat exchanging coil 6I which has an inlet pipe 62 connecting it withthe coils 59 and which is connected with the liquid distributing ring 3lin the digester through the pipe 38. The bottom of the receiver 60 isprovided with a stock outlet pipe 63 which delivers the treated stock toconventional washing and deckering apparatus (not shown) customarilyemployed for producing finished pulp.

As shown in the drawings, the condensing coils 59 are arranged above asump 64 which is adapted to deliver liquid int-o the pipe 65 which isconnected with the weak acid storage tower 66. Intermediate its ends,the pipe 65 is connected through the pipe 61 with a supply tank 6l. Theweak acid storage tower 66 and the receiver 60 are adapted t0 bemaintained under vacuum by means of a vacuum pump 68. The lower portionof the tower 66 is connected by the pipe 69 t0 a liquor pump l0.

In the practice of my process, wood chips or other brous material arefed from the hopper II to the screw I2 which delivers the material intothe space I 6 from which it is discharged into the digester I0 by meansof the screw I3. The pitches of the screws I2 and I3 are so proportionedas to permit compressing of the fibrous material in the space I6 toprovide a seal for the upper end of the digester III. In practice, thetwo screws are separately and independently driven through the shafts I4and I5 in such a manner as to force the brous material into the digesterat the desired rate and at the same time to control the compression ofmaterial in the space I6 so as not to jam the lower screw and preventthe creation of a wholly air-tight seal. It has been found that thematerial in the space I6 may be compressed to such a4 degree as topermit the use of pressures in the digester I0 somewhat higher thanatmospheric pressure and at the same time to permit some discharge ofgases from the digester to prevent the creation of a stagnant conditiontherein. The gases which escape are collected by the device 46 andcarried by the pipe 41 to the condensing coils 48 which deliver thecondensed acid solution through the pipe 49 to the acid reclaiming tank50. The principal supply of acid in the tank 50 is received from thecooking acid storage 53. In order to prevent excessive pressure of gasesand vapors at the top of the digester l0, the cold acid solution (aboutC.) from the tank 50 is delivered by the pump 55 to the heatinterchanger 51 thereby maintaining the upper end of the digester I0relatively cool whereby a substantial proportion of gases and vaporsreaching the upper end of the digester are condensed.

The-brous material under treatment in the digester l0 passes slowly fromthe top to the bottom depending upon the rate at which the treatedmaterial is removed from the bottom thereof. As will be apparent, due tothe outward taper of the digester I6, the material therein moves moreslowly as it approaches the actual cooking zone which is adjacent thelower end of the digester. At this point, the fresh cooking liquorintroduced into the digester from the liquid distributor 31 is broughtinto contact with the fibrous material. However, it will be apparentthat the fresh cooking liquor, which is introduced at approximatelyboiling temperature as hereinafter explained, will be vaporized to acertain extent due to the heating action of the steam coils 39, and theresulting gases will pass upwardly, together with steam from the scraperarm 22, counter-current to the flow of fibrous material. The result isthat the hot gases tend to heat the downwardly moving body of fibrousmaterial so that at its point of contact with the fresh cooking liquorit is at approximately the temperature of such liquor whereby theessential chemical reactions take place most speedily and' with thegreatest 4eiiliciency. Moreover, as the gases and steam pass upwardlythey are absorbed in part by the fibrous material. This is doublyadvantageous in that it reduces the volume of gases discharged from thedigester and also serves to impregnate the brous material with thecooking acid so that when the material reaches the point of directcontact with the fresh cooking acid, the acid concentraton of the brousmaterial approaches the concentration of the fresh acid with the resultthat the cooking is completed in a relatively short time.

The time required for treating a given amount of brous material, andalso the particular temperature of treatment to be employed, will dependupon the particular material to be disintegrated. The temperature ismaintained and controlled by the steam coils 39, the control beingeffected automatically in a conventional manner by the thermostaticdevice 44 which controls the valve 43 in the steam inlet pipe 40.Ordinarily, in a commercial digester of approximately one hundred feetin length the cooking cycle normally requires about nine hours from thetime the fibrous material enters the digester until the pulp isdischarged from the bottom. As indicated above, the temperature withinthe digester will be the lowest at the top thereof and will graduallyincrease toward the bottom. The introduction of steam from the hollowscraper arm 22 serves to bring the temperature of the mass to itsmaximum point although the major proportion of heat input is from thesteam coils 39. The following table indicates an approximate temperatureschedule inside the cooking vessel in a typical operation of theprocess:

0 ft. (top) 30 C.

10 40 C. 20 65 C. 85 C. 100 C. 50 110 C. 60 120 C. '10 125 C. 80 128 C.90 132 C. 100 (bottom) 140 C.

The cooked pulp is continuously removed from the digester I0 through theopening 3!) in the bottom plate I1, the rotating scraper arm 22facilitating the discharging of the pulp from the digester. The masscontaining the pulp is delivered through the pipe 34 to the receiver 60.As pointed out above, a relatively high Vacuum is maintained within thereceiver 60 with the result that when the cooked pulp is discharged intothe receiver from the pipe 34 the cellulose fibers thereof will expandsuddenly. The cavities of the cellulose fibers will be iilled with steamwhen the pulp leaves the bottom of the digester I0 and the iiber wallsthemselves are tender as a result of the cooking action and, hence, whenthe pulp is transferred from the high temperature and high pressurecooking condition, as it exists in the digester, into the receiver 60,the fibers will be subjected to a relatively high vacuum and will expandsuddenly. The resulting fraying eilect upon the fibers has been found tobe particularly beneficial for the manufacture of certain paperproducts.

The blowing off o-f the cooked pulp mass in the receiver 6B results inthe conversion of a large portion of the cooking acid solution intoVapor phase and the resulting acid gases pass upwardly into contact withthe condensing coils 59 which are maintained relatively cool by thepassage therethrough of fresh acid solution from the acid reclaimingtank 50. The resulting condensate is collected by the sump 64 anddelivered therefrom by the pipe 65 to the storage tower 66. As pointedout above, the vacuum pump 68 which is connected to the tower 66 which,in turn, is connected to the receiver 60, mainta' ns the tower and thereceiver under relatively high vacuum. Hence, not only does the liquidcondensed by the coils 59 pass into the tower 66, but also the gasesreleased in the receiver 60 and, that are not condensed, are drawnthrough the pipe 65 into the body of acid solution in the tower 66 andare substantially completely absorbed therein. Obviously, a sufficienthead of acid solution is maintained in the tower 66 in order to assureproper absorption. The recovery of acid is further aided by introducinga milk of lime solution, as is generally employed in the manufacture ofbisuliite liquor, through the pipe 61 into the pipe 65 Where it willcombine with the sulfur dioxid while passing to the storage tower G6.The weak acid from the tower 66 may be pumped by the pump 10 to any ofthe main acid making apparatus (not shown).

As will be apparent from the foregoing, the loss of acid is reduced to apractical minimum by the use of the preferred form of apparatus shown inthe drawings. Moreover, the use of this apparatus reduces heat losses toa practical minimum. For example, the cold acid from the tank 50 isslightly preheated while passing through the heat interchanger 51 andits temperature is increased to a marked degree as it passes throughthe. condensing coils 59, a substantial proportion of the heat from thegases passing over the coils being exchanged thereto. However, it is bythe passage of the acid through the coils 6| that the solution isbrought to boiling temperature, at which temperature it is delivered tothe digester through the distributor 3'1. As will be apparent, asubstantial head of the cooked pulp mass should be maintained in thelower portion of the receiver 60 in order that there may be a maximumexchange of heat therefrom to the acid solution passing through thecoils 6|. Accordingly, the rate of discharge of the stock from thereceiver B through the stock outlet 63 should be controlled so that thehead of cooked pulp mass will be sulicient to substantially completelycover the coils 6l. l

As will be readily apparent to those skilled in the art, the use of myprocess and apparatus results in numerous practical advantages notpossessed by systems heretofore employed. In the first place, continuousproduction of 'pulp is provided with a single digester. In the secondplace, the fibrous material and cooking liquor are brought together whenboth are at a temperature favorable to rapid and efficient cooking. Inaddition, the time of operation and effort in handling materials aregreatly reduced. Moreover, because of the eflicient heat recovery therequirements for heat input are materially reduced. Finally, there ispractically no loss of waste acid gases, thereby effecting a saving inchemicals. In addition to these advantages in operation, my inventionpossesses the important advantage that the pulp produced is ofparticularly high quality and can therefore be marketed at a premium.

While I have described in detail the preferred practice of my processand the preferred form of apparatus to be employed therewith, it is tobe understood that the details of procedure in practicing the processand the size, form, and arrangement of parts of the apparatus may beconsiderably modified without departing from the spirit of the inventionor the scope of the subjoined claims.

I claim:

1. A continuous process of cooking pulp which comprises introducing intoa digester fibrous material to be converted into pulp, such fibrousmaterial being introduced substantially free from pulp cookingchemicals, moving a body of said fibrous material in said digestertoward a pulp cooking zone, elevating the temperature of said fibrousmaterial in said digester during its movement toward said cooking zone,maintaining a pulp cooking temperature in said cooking zone, mixing withsaid body of fibrous material as it approaches said cooking zone cookingliquor at an elevated temperature and in substantially unspentcondition, passing the fibrous material in Contact with said cookingliquor through said cooking zone, and moving the resulting cooked pulpaway from said cooking zone and out of said digester.

2. A continuous process of cooking pulp which comprises introducing intoa digester fibrous material to be converted into pulp, said fibrousmaterial being introduced at substantially atmospheric temperature andsubstantially free from pulp cooking chemicals, moving a. body of saidfibrous material in said digester toward a. pulp cooking zone, elevatingthe temperature of said fibrous material in said digester during itsmovement toward said cooking zone, maintaining a pulp cookingtemperature in said cooking zone, mixing with said body of fibrousmaterial as it approaches said cooking zone cooking liquor atapproximately boiling temperature and in substantially unspentcondition, passing the fibrous material in contact with saidcooking-liquor through said cooking zone, and moving the resultingcooked pulp away from said cooking zone and out of said digester.

3A continuous process of cooking pulp which comprises introducing into adigester fibrous material to be converted into pulp, said fibrousmaterial introduced into said digester being substantially free frompulp cooking chemicals and at a temperature substantially below pulpcooking temperature, moving a body of said fibrous material in saiddigester toward a pulp cooking zone in which a pulp cooking temperatureis maintained, elevating the temperature of said fibrous material as itmoves toward said pulp cooking zone by means of heat discharged fromsaid zone, mixing with said body of fibrous material as it approachessaid cooking zone cooking liquor at an elevated temperature and insubstantially unspent condition, passing the fibrous material in contactwith said cooking liquor through said cooking zone, and moving theresulting cooked pulp away from said cooking zone and out of saiddigester.

4. A continuous process of cooking pulp which comprises introducing intoa digester fibrous material to be converted into pulp, said fibrousmaterial introduced into said digester being substantially free frompulp cooking chemicals and at a temperature below pulp cookingtemperature, moving a body of said fibrous material downwardly in saiddigester toward a pulp cooking zone in which a pulp cooking temperatureis maintained, elevating the temperature of said fibrous material as itmoves toward said pulp cooking zone by means of hot gases and vaporsmoving upwardly from said pulp cooking'zone, mixing with said brousmaterial as it approaches said cooking zone cooking liquor atapproximately boiling temperature and in substantially unspentcondition, passing the fibrous material in contact with said cookingliquor through said cooking zone, and moving the resulting cooked pulpand spent cooking liquor away from said cooking zone and out of saiddigester.

5. In a process of cooking pulp wherein fibrous material to be convertedinto pulp is introduced into a digester, mixed with substantiallyunspent cooking liquor at an elevated temperature, and the mixture offibrous material and cooking liquor is passed through a cooking zone,the improvement which comprises introducing said fibrous material intosaid digester at a temperature below pulp cooking temperature andsubstantially free from pulp cooking chemicals, and elevating thetemperature of said fibrous material in said digester prior to mixing itwith said pulp cooking liquor.

6. In a continuous process of cooking pulp wherein fibrous material tobe converted into pulp is introduced into a digester, mixed with cookingliquor at approximately boiling temperature and in substantially unspentcondition, and the mixture of said fibrous material and cooking liquoris passed through a pulp cooking zone in said digester, the improvementwhich comprises introducing said fibrous material into said digester ata temperature below pulp cooking temperature and substantially free frompulp cooking chemicals, and elevating the temperature of said iibrousmaterial in said digester by means of hot gases and vapors from saidpulp cooking zone to heat said fibrous material to approximately pulpcooking temperature prior to contacting the same with said cookingliquor.

1. A continuous process of cooking pulp which comprises moving a currentof fibrous material to be converted into pulp toward a pulp cooking zonein contact with a counter-current of steam and vapors from heated pulpcooking liquor, mixing cooking liquor at approximately pulp cookingtemperature with said fibrous material as it approaches said cookingzone, maintaining a pulp cooking temperature in said cooking zone,passing the fibrous material in contact with said cooking liquor throughsaid cooking zone, moving the resulting hot cooked pulp away from saidcooking zone, and transferring heat from said hot cooked pulp toadditional amounts of said cooking liquor prior to mixing the same withadditional amounts of said fibrous material.

8. A continuous process of cooking pulp which comprises moving a body offibrous material to be converted into pulp toward a pulp cooking zone,maintaining a pulp cooking temperature in said zone, mixing cookingliquor at approximately boiling temperature with said fibrous materialas it approaches said cooking zone, passing the fibrous material incontact with said cooking liquor through said cooking zone, moving theresulting hot cooked pulp away from said cooking zone, and passing acurrent of said hot cooked pulp in indirect contact with acounter-current of additional amounts of said cooking liquor to transferheat thereto prior to mixing said cooking liquor with additional amountsof said fibrous material.

9. A continuous process of cooking pulp which comprises moving a body offibrous material to be converted into pulp toward a pulp cooking zone,maintaining a pulp cooking temperature in said zone, mixing cookingliquor at an elevated temperature with said fibrous material as itapproaches said cooking zone, passing the fibrous material in contactwith said cooking liquor through said cooking zone, moving the resultingmass of hot cooked pulp and cooking liquor away from said cooking zone,transferring heat from said mass to additional amounts of preheatedcooking liquor prior to mixing the same with additional amounts of saidiibrous material, and preheating said additional amounts of cookingliquor with hot gases and vapors discharged from said mass of hot cookedpulp and cooking liquor.

10. A continuous process of cooking pulp which comprises moving a bodyof fibrous material to be converted into pulp toward a pulp cookingzone, maintaining a pulp cooking temperature in said zone, mixingcooking liquor at approximately boiling temperature with said fibrousmaterial as it approaches said cooking zone, passing the brous materialin contact with said cooking liquor through said cooking zone, movingthe resulting mass of hot cooked pulp and spent cooking liquor away fromsaid cooking zone, and heating additional amounts of said cooking liquorby means of heat released from the hot cooked pulp and spent liquorremoved from said cooking zone and the blow-off gases from said spentliquor.

11. A continuous process of cooking pulp which comprises continuouslycooking pulp with cooking liquor in a pulp cooking zone maintained at apulp cooking temperature, continuously withdrawing cooked pulp and spentliquor from said cooking zone, continuously supplying cooking liquor atapproximately boiling temperature to be mixed with iibrous material tobe converted into pulp, and continuously maintaining said cooking liquorat approximately boiling temperature by means of heat discharged fromthe cooked pulp and spent liquor removed from said cooking zone.

12. A pulp 'cooking apparatus which comprises an elongated digester,means for continuously supplying fibrous material to said digester,means for continuously withdrawing cooked pulp from said digester,heating means in said digester for providing an elevated temperaturetherein, and means adjacent said heating means for introducing cookingliquor into said digester.

13. A pulp cooking apparatus which comprises an elongated digester,means for continuously supplying fibrous material to said digester,means for continuously withdrawing cooked pulp from said digester,heating means in said digester for e providing an elevated temperaturetherein, means adjacent said heating means for introducing cookingliquor into said digester, said heating means being connected to asource of a heating medium, automatic control means arranged betweensaid source and said heating means for regulating the supply of saidheating medium to said heating means, and temperature responsive meansadjacent to said heating means and connected to said automatic controlmeans for regulating the temperature in said digester.

14. A pulp cooking apparatus which comprises a pulp digester, means forcontinuously introducing fibrous material into said digester, means forcontinuously withdrawing cooked pulp from said digester, heating meansfor maintaining an elevated temperature Within said digester, liquidsupply means adjacent said heating means for introducing cooking liquorinto said digester, and means for transferring heat from materialwithdrawn from said digester to the cooking liquor introduced into saiddigester by said liquid supply means.

15. In combination with a pulp digester having a walled inlet forintroducing fibrous material into said digester, a plurality of spacedscrews arranged in said inlet substantially in contact with the wallthereof, said screws being independently rotatable and adapted tocompress brous material therebetween and in contact with the wall ofsaid inlet, one of said screws being adapted to move fibrous materialinto the interior of said digester.

16. In combination with an elongated pulp digester and means for feedingilbrous material to said digester, a pair of spaced screws arranged inthe inlet end of said digester substantially in contact with the wallsthereof, the outer of said screws having a sharper pitch than the innerof said screws, said screws being independently rotatable and adapted tocompress fibrous material therebetween and against the wall of saiddigester, the -inner of said screws being adapted to feed brous materialfrom the space between said screws into the interior of said digester.

1'7. A pulp cooking apparatus which comprises a pulp digester, cookingliquor supply means arranged in said pulp digester, a cooked pulpreceiver, means for conducting cooked pulp from said digester to saidreceiver, a source of cooking liquor, means for preheating said cookingliquor by means of hot gases released in said receiver, means arrangedin said receiver for transferring heat to preheated cooking liquor fromhot masses containing cooked pulp conducted to said receiver from saiddigester, and means for conducting thus heated cooking liquor to saidcooking liquor supply means arranged in said digester..

18. A pulp cooking apparatus which comprises an elongated, verticallyarranged pulp digester, a cooking liquor distributor arranged in saiddigester, a heat interchanger around said digester at the upper endthereof, an elongated, vertically arranged cooked pulp receiver, heatexchange means arranged in the upper portion of said receiver, heatexchange means arranged in the lower portion of said receiver, means forwithdrawing 'a mass of hot cooked pulp and spent cooking liquor fromsaid digester and delivering it to said receiver, a source of cookingliquor, connections for delivering fresh cooking liquor from said sourcethereof to said heat interchanger around the upper end of said digester,connections for delivering the cooking liquor from said Iasi'l namedheat interchanger to the heat exchange means arranged in the upperportion of said receiver, connections for delivering cooking liquor fromsaid last named heat exchange means to the said heat exchange mea'nsarranged in the lower portion of said receiver, and means for deliveringsaid cooking liquor from said last named heat exchange means to saidcooking liquor distributor arranged in said digester.

WILLIAM BRUBACHER.

